Welcome, VIRGO! Another merger of two big black holes has been detected, this time by both LIGO’s two detectors and by VIRGO as well.
Aside from the fact that this means that the VIRGO instrument actually works, which is great news, why is this a big deal? By adding a third gravitational wave detector, built by the VIRGO collaboration, to LIGO’s Washington and Louisiana detectors, the scientists involved in the search for gravitational waves now can determine fairly accurately the direction from which a detected gravitational wave signal is coming. And this allows them to do something new: to tell their astronomer colleagues roughly where to look in the sky, using ordinary telescopes, for some form of electromagnetic waves (perhaps visible light, gamma rays, or radio waves) that might have been produced by whatever created the gravitational waves.
The point is that with three detectors, one can triangulate. The gravitational waves travel for billions of years, traveling at the speed of light, and when they pass by, they are detected at both LIGO detectors and at VIRGO. But because it takes light a few thousandths of a second to travel the diameter of the Earth, the waves arrive at slightly different times at the LIGO Washington site, the LIGO Louisiana site, and the VIRGO site in Italy. The precise timing tells the scientists what direction the waves were traveling in, and therefore roughly where they came from. In a similar way, using the fact that sound travels at a known speed, the times that a gunshot is heard at multiple locations can be used by police to determine where the shot was fired.
You can see the impact in the picture below, which is an image of the sky drawn as a sphere, as if seen from outside the sky looking in. In previous detections of black hole mergers by LIGO’s two detectors, the scientists could only determine a large swath of sky where the observed merger might have occurred; those are the four colored regions that stretch far across the sky. But notice the green splotch at lower left. That’s the region of sky where the black hole merger announced today occurred. The fact that this region is many times smaller than the other four reflects what including VIRGO makes possible. It’s a small enough region that one can search using an appropriate telescope for something that is making visible light, or gamma rays, or radio waves.
While a black hole merger isn’t expected to be observable by other telescopes, and indeed nothing was observed by other telescopes this time, other events that LIGO might detect, such as a merger of two neutron stars, may create an observable effect. We can hope for such exciting news over the next year or two.